In Ethernet networks that operate over shared copper media, e.g., IEEE 802.3 standards for 10Base5, 10Base2 and 10BaseT, apparatus in the network access device for a connected terminal typically includes a so-called xe2x80x9cjabberxe2x80x9d circuit that monitors the length of transmission into the copper media for that individual terminal. If the continuous transmission of the individual terminal exceeds an accepted maximum length of time many times a maximum size packet, that terminal is disabled for a quiet period of, e.g., 500 milliseconds and then usually is allowed to resume transmission.
In a shared transmission medium such as a passive optical network which is shared among multiple terminals, it is important for proper operation that each terminal transmit onto the shared medium at assigned times and only for the assigned time interval. If such rules of operation are violated, the integrity of the system transmission can be severely compromised.
There is a need for a mechanism for protecting access to a shared optical medium in order to ensure that a faulty terminal does not affect system operation and integrity.
The present invention provides an apparatus and method for protecting access to a shared transmission medium. According to one aspect, if an optical transmitter for an individual terminal is active for a first prescribed time interval when it has not been enabled for such transmission, the optical transmitter is disabled. According to another aspect, the continuous time that an individual terminal is enabled to transmit onto the shared medium is monitored. If the enabled transmission for the individual terminal exceeds a second prescribed time interval, the optical transmitter for the individual terminal is disabled.
Accordingly, in an optical communications system which has plural remote terminals coupled to a central terminal or hub over a shared optical transmission medium, wherein the plural remote terminals are configured to transmit data on the shared optical transmission medium at prescribed transmission time intervals and in a prescribed sequence, a remote terminal apparatus comprises an optical transmitter for transmitting data onto the shared optical transmission medium, a fault monitoring circuit and a deactivation circuit. The fault monitoring circuit is coupled to the optical transmitter for monitoring activity of the optical transmitter and providing a fault indication signal when optical transmitter activity occurs for a duration that exceeds a fault time interval. The deactivation circuit is responsive to the fault indication signal to deactivate the optical transmitter.
In a first embodiment, the optical transmitter includes an optical detector for detecting the presence of optical transmitter energy into the shared medium to provide an activity signal to the fault monitoring circuit which operates to provide the fault indication signal upon the activity signal being present for a duration that exceeds the fault time interval.
In another embodiment, the remote terminal apparatus includes a transmitter controller that provides a transmit enable signal for enabling the optical transmitter wherein under normal operation of the remote terminal the transmit enable signal is asserted during the prescribed transmission time interval. The optical transmitter includes an optical detector for detecting the presence of optical transmitter energy into the shared medium to provide an activity signal to the fault monitoring circuit which operates to provide the fault indication signal upon the transmit enable signal being unasserted and the activity signal being present for a duration that exceeds the fault time interval.
According to an aspect, the fault monitoring circuit includes a timer circuit comprising a resistance-capacitance time constant circuit having an input coupled to the activity signal for establishing the fault time interval which can be less than the prescribed transmission time interval.
According to another aspect, the remote terminal apparatus further includes a second fault monitoring circuit for monitoring the transmit enable signal to provide a second fault indication signal upon the transmit enable signal being asserted for a duration exceeding a second fault time interval. The deactivation circuit includes a fault latch for latching the first and second fault indication signals for deactivating the optical transmitter.
According to yet another aspect, the second fault monitoring circuit includes a second timer circuit comprising a voltage ramp generator that has an input coupled to the transmit enable signal to provide a ramp voltage and a voltage comparator that has a first input coupled to the ramp voltage and a second input coupled to a reference voltage. The voltage comparator is operable to provide the second fault indication signal upon the ramp voltage exceeding the reference voltage over the second fault time interval. The second fault time interval is greater than the prescribed transmission time interval.
A method of the present invention includes providing an optical transmitter for transmitting data onto a shared optical transmission medium; monitoring activity of the optical transmitter output and providing a fault indication signal upon optical transmitter activity occurring for a duration that exceeds a fault time interval; and deactivating the optical transmitter responsive to the fault indication signal.